The present specification relates to displays, such as, head up displays (HUDs). More particularly, the present specification relates to displays with increased optical efficiency.
Displays, such as HUDs, can be used in a variety of applications. In aircraft applications, HUDs can provide significant safety and operational benefits including precise energy management and conformal flight paths. These safety and operational benefits are enjoyed by operators of air transport aircraft, military aircraft, regional aircraft and high end business jets where HUDs are generally employed. These safety and operational benefits are also desirable in smaller aircraft.
Substrate guided HUDs have been proposed which use waveguide technology with diffraction gratings to preserve eye box size while reducing lens size. U.S. Pat. No. 4,309,070 issued to St. Leger Searle and U.S. Pat. No. 4,711,572 issued to Upatnieks disclose substrate guided HUDs. However, such systems have faced difficulties in design.
Substrate guided HUDs have been proposed which provide pupil expansion using transparent diffraction gratings for in-coupling light from collimating optics and out-coupling light to the pilot at the output of the waveguide. The light directed toward the pilot is diffracted in a reflective manner by the out-coupling diffraction grating. However, a considerable amount of diffracted light is also transmitted through the out-coupling diffraction grating in a direction that it is opposite to the desired direction toward the pilot. This transmission of diffracted light through the diffraction grating in a direction that it is opposite to the desired direction lowers the optical efficiency of the HUD.
In addition, the diffraction gratings employed by substrate guided HUDs can be fragile and delicate. Diffraction gratings can be damaged by debris or improper handling.
Thus, there is a need to protect the output diffraction grating in a substrate guided HUD. Further, there is a need for increasing the optical efficiency of a substrate guided HUD. Still further, there is a need for a HUD including a substrate waveguide configured for pupil expansion with acceptable optical characteristics. Yet further, there is a need for a HUD with pupil expansion which enables a wide field of view and acceptable luminance. Further still, there is a need for a low cost HUD including diffraction gratings that meet performance and cost requirements. Yet further still, there is a need for a compact HUD which uses a configuration optimized for protection of at least one diffraction grating and optical efficiency. Yet further still, there is also a need for a small volume, lightweight, lower cost HUD with superior optical efficiency.